7-well delta pattern for secondary recovery



April 30, 1968 A. F. ALTAMIRA ET AL 3,380,525

7-WELL DELTA PATTERN FOR SECONDARY RECOVERY Filed June 28, 1966 5 Sheets-Sheet 1 Tljla.

April 30, 1968 ALTAMlRA ET AL 3,380,525

7-WELL DELTA PATTERN FOR SECONDARY RECOVERY 5 Sheets-Sheet 2 Filed June 28, 1966 07, ww A O@ April 30, 1968 ALTAM|RA ET AL 3,380,525

7-WELL DELTA PATTERN FOR SECONDARY RECOVERY Filed June 28, 1966 5 Sheets-Sheet 5 April 30, 1968 A. F. ALTAMIRA ET AL 3,380,525

7-WELL DELTA PATTERN FOR SECONDARY RECOVERY Filed June 28, 1966 5 Sheets-Sheet 4 April 30, 1968 A. F. ALTAMIRA ET 3,330,525

WELL DELTA PATTERN FOR SECONDARY RECOVERY 5 Sheets-Sheet 5 Filed June 28, 1966 United States 3,380,525 7-WELL DELTA PATTERN FOR SECONDARY RECQVERY Anthony F. Altamira and Donald L. Hoyt, Houston, Tex.,

assignors to Texaco Inc., New York, N.Y., a corporation of Delaware Filed June 28, 1966, Ser. No. 561,110 10 Clain. s. (Cl. 166-9) and the production of hydrocarbons is started and carried,

out at other production wells in the field. It is known that in such instances, a substantial amount of hydrocarbons is left behind in the hydrocarbon-bearing formation since such is not considered primarily recoverable economically.

Secondary recovery operations are now an essential part of the over-all program planning for virtually every oil and gas-condensate reservoir in underground hydrocarbon-bearing formations. Some of the operations developed include gas repressuring and water, fire, steam and solvent flooding. Usually, they all employ some geometric pattern of injection and production wells, with injection of fluid into some of the wells to displace hydrocarbons in the reservoir zone toward the production wells.

The front, or interface, between the injected and in-place fluids moves from injection toward production wells, changing shape as it progresses. Due to the pressure sinks around the production wells, a portion of the interface tends to accelerate and cusp into the production wells. Breakthrough of the injected fluid occurs when the interface reaches the production wells. The percentage of the entire reservoir which has been invaded by the injected fluid at that time is referred to as the sweep efliciency of the particular geometric pattern used.

The most commonly used well pattern in secondary recovery is the S-spot pattern, with four injection wells at the corners of a square and a production well at the center. In a production field of S-spot patterns, there are as many injection wells as production wells. Sweep efiiciency for the S-spot pattern is about 71%. Other basic flood patterns sometimes used are the 7-spot, direct line and staggered line drives, with sweep efliciencies of 74, 57 and 78 percent respectively.

In field practice, injection usually will be continued well past breakthrough until the reservoir cannot be produced economically. In this way, some additional sweepout can be achieved, but often there will be large volumes of produced injection fluid to be handled, treated and reinjected. If sweepout prior to breakthrough in a pattern flood were improved, it is very likely that the ultimate recovery would be higher. The time and cost of the operation, to achieve comparable recoveries, would be reduced accordingly.

Accordingly, it is an object of the present invention to provide an improved method for the production and recovery of hydrocarbons, particularly liquid petroleum, from underground hydrocarbon-bearing formations.

Another object of this invention is to provide a method whereby the areal sweep efliciency in pattern flooding is improved.

These and other objects, advantages and features of this invention will become apparent from a consideration of the specification with reference to the figures of the accompanying drawings wherein:

FIGS. 1a and lb disclose respectively conventional 5-spot and inverted S-spot patterns in a field undergoing secondary recovery, illustrating the interface of the injected fluid at breakthrough at the production wells;

FIG. 10 discloses a staggered line pattern, illustrating the interface of the injected fluid at breakthrough at the production wells in a field undergoing secondary recovery;

FIG. 2 discloses the formation of a 7-well delta pattern in a field of staggered, equidistantly spaced wells;

FIGS. 3 and 4 illustrate the movements of the interface of the injected fluid during the two phases of the exploitation plan in one 7-well delta pattern unit in a field undergoing secondary recovery;

FIG. 5 discloses the flow pattern of a section of symetry, outlined by the dashed lines in FIG. 4, showing the advance of the interface of the injected fluid during the second phase of the plan and the formation of the central petal-shaped unswept area.

FIGS. 6a and 6!) illustrate the movements of the interface of the injected fluid during the two phases of an alternate exploitation plan in one 7-well delta pattern unit, similar to the disclosures in FIGS. 3 and 4; and

FIGS. 7a and 7b correspond to FIGS. 6a and 6b, illus trating the interface movements of another alternate exploitation plan.

In our copending, coassigned application for patent Ser. No. 517,052, filed Dec. 28, 1965, for Interface Advance Control in Pattern Floods by Use of Control Wells, there is disclosed how an increased amount of hydrocarbons is produced and recovered from an underground hydrocarbon-bearing formation by employing at least three wells, penetrating such a formation, which wells can be in-line, to produce hydrocarbons from the formation via two of these wells including the middle well, as disclosed in the coassigned US. Patent No. 3,109,487, issued to Donald L. Hoyt, on Nov. 5, 1963.

It is understood that the failure of the driving flood in secondary recovery operations to contact or sweep all the hydrocarbon area is due to the development, in the interface, of a cusp which advances toward the production well. If other portions of the interface could be made to keep up, or if the cusp formation were delayed, complete areal sweep would be possible. In the above cited coassigned application, a production control well is positioned between the injection well and the production well and is kept on production even after the injection fluid reaches it. In this manner, the cusp is pinned down at the control well and while the area swept out by the injection fluid before breakthrough at the last production well is increased, there is an unwanted handling of considerable quantities of injection fluid at the control well.

Another aspect to increase the sweepout efiiciency is disclosed in our copending, coassigned application for Patent Ser. No. 516,891, filed Dec. 28, 1965 for Interface Advance Control in Pattern Floods by Retarding Cusp Formation, and involves the retardation of the development of the cusp toward the production well. The method of achieving more uniform advance is to control the flow gradients so that the interface is spread out. This can be done either by choosing a particular geometry of well positions or by adjusting the relative production rates so that the velocity of advance is not predominantly in one direction. It can be done also by shifting the gradients frequently, in both direction and magnitude, thus preventing any one section of an interface from advancing too far out of line.

The figures of the drawings schematically disclose and illustrate the practice and the advantages of our invention with well pattern and areal sweepout examples which are obtainable and have been observed both in secondary reeovery operations and in potentiometric model studies which simulate secondary recovery operations. The model studies indicate a sweepout obtained in an ideal reservoir, although the recovery from an actual sweepout of a particular field may be greater or less, depending on field parameters. The procedures to be described are based on the following set of experimental conditions and assumptions:

(1) All the units in any pattern are balanced and produce at the same rate. This requires that Wells on the edges of a pattern unit produce or inject at /6 or /2 of the rate of interior wells, depending inversely on the number of pattern units with which they are associated;

(2) The total amount of fluid injected must be equal to the fluid produced for each pattern unit, as well as for the whole pattern;

(3) The mobility ratio of the displacing to the displaced fluid is 1.0;

(4) The permeability and sand thickness of the formation is uniform; and

(5) Gravitational effects are not considered.

Throughout the drawings, except in FIG. 2, the same symbolic indicators will be maintained as follows: P P and P, represent respectively wells at the corners, along the sides, and the interior or middle wells; and, a solid circle indicates a production well, a crossed circle indicates a shut-in well, and an arrowed circle indicates an injection well. In FIG. 2 a single circle indicates an original field well, and a double circle indicates an additional well to be drilled.

FIG. 1a illustrates the interface of the injected fluid at breakthrough at the production well of a single conventional S-spot pattern unit in a production field undergoing secondary recovery, wherein the corner wells of each pattern unit are injection wells, while the inner central well is used for production.

FIG. lb illustrates the interface of the injected fluid at breakthrough at the production wells of a unit of an inverted S-spot pattern, wherein the secondary recovery flooding fluid is injected into the central well and production is maintained at the corner wells until breakthrough thereat. Such a procedure will produce a sweepout of approximately 71%. In both FIGS. 1a and lb, the ratio of the distance between the rows of wells to the distance between the in-line wells is 0.5, i.e., d/rt /z.

FIG. 1c illustrates the interface of the injected fluid at breakthrough at the production wells of a staggered line pattern, wherein the wells are spaced apart in rows a distance equal to the distance between rows with alternate rows of wells being production and injection wells r respectively, so that d/a=1.

Referring to FIG. 2, there is disclosed the formation of a 7-well delta pattern comprising six wells spaced equi distantly from each other at the vertices of an equilateral triangle and midway between these wells on the sides of this triangle, and a single interior or middle well. The wells on the sides and at the vertices are part of a staggered equidistant well spacing, the ratio of the distance between the rows of wells to the distance between the in-line Wells being 0.866, i.e., d/a=(l.866.

FIGS. 3 and 4 disclose the consecutive formations of the interface of the driving fluid during a two phase secondary recovery exploitation plan as would occur in one 7-well delta pattern unit.

FIG. 3 discloses the first phase of the interface formation wherein driving fluid is injected into the wells at the corners or the vertices of an equilateral triangle, with production at the equidistan'tly spaced Side walls until breakthrough of the injected fluid occurs thercat. At this breakthrough, a substantially equilateral triangular area defined by the cusp formations ending at the production side wells remains, as indicated by cross-hatching in FIG. 3, representing an areal sweepout of 62%. The ratio of the injection well rate to the production well rate is 3 to 1, since there are three production wells for each injection well. Both injection and production rates are uniformly distributed.

FIG. 4 discloses the second phase of the exploitation lan, wherein an interior or middle well, indicated as P drilled at the median point of the equilateral triangle defined by the three corner and three side wells to complete the 7-well delta pattern, is put on production. The corner wells, which served as injection wells in the first phase, are now shut in, and the side wells, serving as pro duction wells in the first phase, are converted to injection wells, the symbolic indicators being used as noted previously. A additional 32% sweepout is attained at breakthrough of the injected fluid into the production well, to give a total of 94%. sweep efiiciency. The ratio of the injection well rate to the production well rate is 2 to 3, since there are two production wells for each three injection wells. Both the injection and production rates are uniformly distributed.

The unswept area at the end of the second phase, as disclosed in FIG. 4, is petal-shaped, clustered around the production well within a drainage radius which encompasses only 22% of the total pattern, and in a region with the strongest flow gradient. This is a significant advantage since, in many patterns, much of the unswept area is far from the production well, and in the region of such low flow gradient as to be effectively inaccessible.

FIG. 5 discloses, in a section of symmetry as indicated in FIG. 4, the flow pattern during the second phase of the exploitation plan, showing the advance of the interface and the formation of the finally remaining petalshaped unswept areas. The nomenclature on this figure is self-explanatory of the disclosure thereof.

In this type of exploitation plan, the additional drilling of the interior or middle production well can be deferred until the end of the first phase. However, where there is an additional interior or middle well in being, or should this well be drilled as part of the overall exploitation plan of the production field, then substitute exploitation plans, as disclosed in FIGS. 6a and 6b, and 7a and 712, may be used to increase the sweepout elficiency over that for other conventional patterns, e.g. the 5-spot pattern.

Referring to FIG. 6a, wherein the first phase of an alternate exploitation plan is disclosed, the corner wells are shut in, injection is maintained at the interior or middle well, with the side Wells on production until breakthrough of the injected fluid thcreat, to result in a substantially triangular central swept out area, the cross-hatching indicating the area remaining for further sweepout. In the second phase of this alternate exploitation plan as dis closed in FIG. 6b, the interior or middle injection well is shut in, injection is commenced at the side wells, and production is initiated at the corner wells until breakthrough of the injected fluid thereat. A total sweepout of approximately 91.7% is obtained, more than 20% greater than that for the 5-spot pattern.

FIGS. 7a and 7b disclose another alternate exploitation plan. FIG. 7a shows the first phase of this alternate exploitation plan wherein injection is initiated at the interior or middle well, with production at both corner and side Wells until breakthrough of the injected fluid occurs at the side wells, also resulting in a substantially triangular, central swept out area, the cross hatching indicating the area remaining for further sweepout.

FIG. 7b discloses the second phase of this alternate exploitation plan, with the interior well shut in, and injection initiated at the side wells, while production of the corner wells continues until breakthrough of the injected fluid. The overall sweepout is calculated to be approximately 90.7%.

As in the case of the preferred exploitation plan as illustrated in FIGS. 3 and 4, the unswept areas are located around the production wells, in areas having the strongest flow gradients. Thus, further production may be continued after breakthrough with comparatively less production of the injected fluid than in the ordinary production following breakthrough.

It is recognized that any of the more than 90% sweepouts obtained by the use of 7-well delta pattern exploited as disclosed herein is an idealization, as in the case for the 71% for the S-spot pattern. None of the values is likely to be achieved in the field, but the relative superiority of the sweepouts obtained by the use of the 7-well delta pattern is clear and is independent of inhomogeneities.

The basic technique employed to increase sweep efficiency controls the advance of the front in the pattern to achieve large areal coverage, by shifting and adjusting the geometric position, direction and magnitude of the flow of pressure gradients. If the positions of the wells in the pattern are favorable, by changing the wells used for injection and production, high flow gradients can be made to cover most of the pattern. The patterns and procedures disclosed herein have high sweep efliciencies leaving the unswept areas at breakthrough in the regions of strong flow gradients adjacent the production wells, and thus readily swept by production past breakthrough.

Although the 7-well delta pattern was developed from a staggered equidistant well spacing, if a staggered line pattern of wells with a d/a ratio of about 1, more or less, has been drilled already, production therefrom should be sufiiciently similar to that from the equidistant well spacing, that all the preferred patterns and procedures disclosed for the 7-well delta pat-tern with equidistant Well spacing can be used to advantage.

The sweepout and volumes of injected fluid produced vary from one pattern to another, and also appear to be functions of rate distribution and distance parameters within a given pattern.

In evaluating the performance of a flood pattern, or in comparing performances of different patterns, there are three main considerations:

(a) Percentage of sweep (b) Volume of injection fluid handled (c) Time to achieve the sweep For a given total field production rate, it will not be possible generally to obtain an increase in sweepout without at least a proportionate increase of time, which is to be expected. However, if the extra time involved is disproportionately long, the gain in sweepout may not be economically worthwhile.

Any pattern and/ or rate distribution which retards the development, or the advance, of a cusp towards the production wells will increase the sweepout of a field. As mentioned previously, two principal means of doing this are: (a) pinning down the cusp by locating production wells between the injection source and the outer production wells, and keeping these inner (or control) wells on production after breakthrough, and (b) spreading out the cusp by pulling the front toward side wells until breakthrough thereat before allowing it to proceed toward the corner production wells of a pattern unit, as disclosed in our above cited coassigned applications.

The spreading out of the cusp is in general a more advantageous procedure. It yields as good or better sweepout with less production of injection fluid. Further, a higher rate distribution on corner wells of pattern units generally results in much less overall production of injection of injection fluid, but also in less sweep (although exceptions can be found in more complicated patterns).

The advantages of the methods disclosed above are evident. Fewer injection wells are required, more reservoir fluid is recovered prior to breakthrough of injection fluid, and so more ultimate recovery is obtained, as compared with other methods generally employed in secondary recovery' operation.

Although emphasis has been placed in this disclosure on the practice of this invention as directed to a secondary recovery operation, particularly employing water or other similar aqueous fluid as the injection fluid or displacement fluid, the advantages obtainable in the practice of this invention are also realized in primary hydrocarbon production operations wherein the hydrocarbon-bearing forma tion is under the influence of a water drive or gas drive, or both a Water and a gas drive, and also in the instance of a secondary recovery operation wherein a gas, such as natural gas, is employed as the injection fluid.

As will be apparent to those skilled in the art, in the light of the accompanying disclosure, other changes and alternatives are possible in the practice of this invention without departing from the spirit or scope thereof.

We claim:

1. A method of producing hydrocarbons from an underground hydrocarbon-bearing formation involving wells located at the vertices and on the sides of a triangle and a well located therewithin which comprises introducing fluid into said formation via the Wells located at said vertices of said triangle, producing hydrocarbons from said formation via the wells located on said sides of said triangle until breakthrough thereat of said fluid introduced into said formation, and thereupon respectively ceasing introducing fluid and producing hydrocarbons via said wells at said vertices and on said sides, and commencing introducing fluid into said formation via the side wells and initiating producing hydrocarbons via said well located within said triangle.

2. In the method of producing hydrocarbons as defined in claim 1, the rates of introducing fluid into and producing hydrocarbons from said formation being controlled so that breakthrough of the introduced fluid at said production wells occurs simultaneously.

3. In the method of producing hydrocarbons as defined in claim 1, said wells located at said vertices of said triangle defining an equilateral structure, and said well located therewithin being positioned equidistantly from said vertices.

4. In the method of producing hydrocarbons as defined in claim 3, said wells located on said sides of said triangle being spaced equidistantly from said vertices whereby said wells at said vertices and on said sides are spaced equidistantly from each other therealong.

5. In the method of producing hydrocarbons as defined in claim 4, said wells being located equally at said vertices of and on said sides of and within said triangle and being at least seven in number, defining a pattern unit in a production field, the wells located at said vertices and on said sides of said triangle being spaced apart and aligned in rows so that the ratio of the distance between the rows of wells to the distance between the in-line wells is 0.866.

6. The method of producing hydrocarbons from an undenground hydrocarbon-bearing formation involving wells located at the vertices and on the sides of a triangle and a well located therewithin which comprises injecting fluid into said formation through said well located within said triangle and producing formation hydrocarbons from the wells situated on the sides of said triangle until breakthrough of the injected fluid thereat, and thereupon ceas ing injecting fluid into said formation through said well within said triangle and ceasing producing formation hydrocarbons from the side wells, and thereafter injecting fluid into said formation through said side wells and producing formation hydrocarbons from the wells at said vertices until breakthrough of injected fluid thereat.

7. In the method of producing hydrocarbons as defined in claim 6, the wells at the vertices and on the sides of said triangle being spaced equidistantly from each other therealong, the well within said triangle being located equidistantly from said vertices of said triangle.

8. In the method of producing hydrocarbons as defined in claim 6, the wells on said sides of said triangle being located intermediate the wells located at said vertices of said triangle, and the well within said triangle being located intermediate said wells at the vertices thereof, to define a 7-well delta pattern unit in a production field.

9. In the method of producing hydrocarbons as defined in claim 6, the step of simultaneously producing formation hydrocarbons from the wells located at said vertices and on said sides of said triangle until breakthrough of the injected fluid at the side wells of said triangle and thereafter ceasing producing formation hydrocarbons therethrough, and commencing injecting fluid into said formation through said side wells while maintaining producing formation hydrocarbons from the wells at said vertices until breakthrough of injection fluid thereat.

10 In the method of producing hydrocarbons as defined in claim 9, the wells located at said vertices and on said sides of said triangle being spaced apart and aligned in rows so that the ratio of the distance between the rows and defining a pattern unit in a production field.

8 References Cited UNITED STATES PATENTS 5/1959 Ienks 166-9 12/1963 Dew et al 166-9 12/1963 Oakes 166-9 12/1963 Oakes 166-9 2/ 1964 Santourian 166-9 8/1964 Foulks 166-9 9/1965 Foulks 166-9 CHARLES E. OCONNELL, Primary Examiner.

of wells to the distance between the in-line wells is 0.866, 15 BROWN, Examiner- 

1. A METHOD OF PRODUCING HYDROCARBONS FROM AN UNDERGROUND HYDROCARBON-BEARING FORMATION INVOLVING WELLS LOCATED AT THE VERTICES AND ON THE SIDES OF A TRIANGLE AND A WELL LOCATED THEREWITHIN WHICH COMPRISES INTRODUCING FLUID INTO SAID FORMATION VIA THE WELLS LOCATED AT SAID VERTICES OF SAID TRIANGLE, PRODUCING HYDROCARBONS FROM SAID FORMATION VIA THE WELLS LOCATED ON SAID SIDES OF SAID TRIANGLE UNTIL BREAKTHROUGH THEREAT OF SAID FLUID INTRODUCED INTO SAID FORMATION, AND THEREUPON RESPECTIVELY CEASING INTRODUCING FLUID AND PRODUCING HYDROCARBONS VIA SAID WELLS AT SAID VERTICES AND ON SAID SIDES, AND COMMENCING INTRODUCING FLUID INTO SAID FORMATION VIA THE SIDE WELLS AND INITIATING PRODUCING HYDROCARBONS VIA SAID WELL LOCATED WITHIN SAID TRIANGLE. 